An F.T.C. spokesman did not immediately respond to a request for comment on the planned lawsuit to stop Microsoft from closing the Activision purchase. MLex, a regulatory news outlet, earlier reported the F.T.C.’s plans.

In a statement, Brad Smith, the president of Microsoft, said the company welcomed the “opportunity to present our case in federal court.”

Microsoft has appealed the British ruling to block the Activision deal and has pledged to fight the F.T.C. in court. But the legal challenges could significantly delay the closing of the deal, which the two companies had said they hoped would occur by July.

The British portions of the deal cannot close while Microsoft’s appeal moves forward. If a federal court grants the F.T.C.’s demand to stop the deal from closing, it would also prevent Microsoft from completing the transaction in the United States.

But if Microsoft does usher the deal through, it would be a monumental moment for the $184 billion gaming industry. Microsoft earns billions of dollars each year in the video game business, but competitors like Nintendo and Sony — which makes the PlayStation console — have long been considered to have a better catalog of games that attract players to their devices. Adding Activision and its slate of gaming studios to Microsoft’s Xbox consoles and its games subscription service could even the playing field.